


i'm only curious (how one can equal two)

by undertailsoulsex



Category: Undertale (Video Game)
Genre: Begging, Cunnilingus, Dubious Consent, Ecto-Genitalia (Undertale), Ecto-Penis (Undertale), Ecto-Tongue (Undertale), Ecto-Vagina (Undertale), Emotional Manipulation, First Time, Fontcest, Incest, Kissing, M/M, Masturbation, Penetration, Rape/Non-con Elements, Rough Kissing, Rough Sex, Semi-Public Sex, Sibling Incest, Smut, To Be Edited, Vaginal Sex
Language: English
Status: Completed
Published: 2020-04-21
Updated: 2020-04-21
Packaged: 2021-03-02 02:14:33
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 4,038
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/23767528
Author URL: https://archiveofourown.org/users/undertailsoulsex/pseuds/undertailsoulsex
Summary: Papyrus is curious.What makes his twin brother look so drastically different from him?Why does Sans have such a hard time at school when he’s the one tutoring Papyrus?How does he make it look so easy to tell those puns, to make those friends, to attract attention wherever he goes?What would it feel like to kiss him?(NOTE: Archive warnings were not used, that doesn't mean they don't apply!)
Relationships: Papyrus/Sans (Undertale)
Comments: 13
Kudos: 164





	i'm only curious (how one can equal two)

**Author's Note:**

  * For [Ganzooky](https://archiveofourown.org/users/Ganzooky/gifts).



> This is unedited and probably will remain so just because I don't know if it's worth it hahaha I kinda need to just put it out there so I can finally write again!!!
> 
> Happy birthday to Ganzooky! You're one cool dude

Papyrus is curious.

His natural curiosity is something every monster likes to remark upon. That, paired with his downright determination, makes him a force to be reckoned with. They may not think him particularly smart, but curiosity? He has that in spades.

Curious about what depends on who you ask. His teachers might say he’s curious about the inner workings of the world, from math to language to puzzles. His peers may think him curious about social interactions, with his constant barrage of questions about why they thought this was funny, or what was friendship like, or how a baseball team is able to function cohesively both individually and as a group. His father might, between meetings, jokingly reply that he is only curious about what’s for dinner.

But if the question were to be asked of Papyrus, which of course it would not, he would happily answer that he was most curious about his brother.

What makes his twin brother look so drastically different from him?

Why does Sans have such a hard time at school when he’s the one tutoring Papyrus?

How does he make it look so easy to tell those puns, to make those friends, to attract attention wherever he goes?

What would it feel like to kiss him?

This last question is the one that causes Papyrus’s face to burn with embarrassment and guilt whenever he thinks it. If anyone is around to see, he excuses himself with a mumbled apology to calm himself down.

Today the question is burned into his mind as Sans sits as his desk, his eyes half-lidded as he’s staring into space, lost in some distant thought. There’s something about the way he’s tracing his finger along his desk, or maybe it’s the way his smile is not quite formed. Whatever it is, he looks absolutely _kissable_ right now and it’s enough to make Papyrus’s thoughts shudder to a halt.

In the middle of a presentation. The class is looking at him, some bored, some expectant for more information on how a burrito is constructed. There’s no room to flee as usual, and yet he can’t do anything but sputter out some incoherent sentences. As he bumbles his way through, his eyes train on Sans. His brother seems to realize he’s being watched and pulls himself from his reverie. That’s when Papyrus is really in trouble. His signature smile curls across his face as he gives a thumbs up. The encouragement only serves to fluster Papyrus more. His tongue fails to form words, and after thirty seconds of stark silence, his teacher mistakenly thinks the presentation is over and leads the classroom in a round of quiet applause.

There’s some giggling as Papyrus returns to his seat and later when the school day ends, though nothing as antagonistic or hurtful as previous incidents. He can only be thankful that high school will soon be over so he can properly avoid his classmates.

But there’s no eluding Sans, try as he might. When the class is released, instead of meeting up for their daily walk home, he darts ahead and heads straight to the house. He doesn’t even stop at the shop for his afternoon snack.

However, there’s not really an easy way to avoid one’s brother when he is a master of teleportation magic.

“yo,” Sans calls out from the couch as Papyrus steps into the house. He’s already turned on the television and their favorite weekday cartoon is spraying rainbows across the screen. “you joining?”

Papyrus wants to say no, that he is tired and needs to rest, but he doesn’t have the nerve, not with Sans. He silently drops his bookbag to the ground, takes off his shoes, and sits down. Sans is either oblivious to his failed escape attempt or has the good grace not to mention it. Not much gets past his brother, so Papyrus suspects the latter.

The first few minutes are tense, or at least they are to Papyrus as he tries to think of some excuse for his behavior. But as it becomes more obvious that Sans isn’t going to say anything, he leans back into the couch. His anxiety takes a turn, twisting from embarrassment to… a different feeling. He isn’t watching the show, but is instead watching Sans out of the corner of his eyes. He’s got that same half-smile from earlier, that half-lidded look. Papyrus’s soul churns with some indescribable feeling, and he again wonders what it would be like to kiss him.

Sans chuckles at some unknown joke on the show. Papyrus belatedly joins him, but Sans doesn’t seem to notice. He’s still watching the TV, his right hand absentmindedly dancing along the couch cushion. There’s something about the motion that enraptures Papyrus. Maybe it’s how slender Sans’s phalanges are – a distinct departure from his thick bones and sturdy, rotund figure – but Papyrus simply wants to put his own fingers on top of them. How would they feel? Were they scarred and nicked like his?

His curiosity gets the better of him.

Sans jumps a little when Papyrus begins to rub his hand. Papyrus can see the confusion in his eyes as he glances over. But he doesn’t say anything, not even as Papyrus brings his finger down from the tip of Sans’s middle phalange, over the crests of his knuckles, and down the long, open expanse of his carpals and metacarpals. It’s delightfully smooth, and Papyrus savors the way it feels so much that he makes the return journey back up the hand.

His eyes dart toward Sans. He’s returned his attention to the television show, though his eyelights aren’t quite focused on the screen and there’s a soft blush spreading across his cheekbones. Still, he isn’t saying anything, and Papyrus rubs small circles into Sans’s hands for a couple minutes before his nerves (and tremendous guilt) get the best of him. He wordlessly withdraws his hand.

The tension is back, and this time Papyrus is quite sure they are both feeling it. Yet neither of them are speaking or moving. They don’t say a word to each other until their father returns home – very early, by his standards – and then the spell is broken, and they are back to their usual trading of puns and quips and fun.

*************************************************************

Sans’s face dances in Papyrus’s mind as he furiously pumps his length. With the lubricant, it’s making slick sounds, but they are not nearly as loud as Papyrus’s moans. He smashes his face into his pillow, hoping more than anything that Sans can’t hear him from the shower. Even though in all his dreams, he hopes that his brother _does_ hear him. Sans could be the one rubbing him up and down, faster, faster, faster –

Papyrus releases into his hands, his brother’s name spilling equally as sloppy into the pillow. It takes a few moments for him to collect his breath, and when he does, it’s disgust, remorse, and horror that fill his soul. Cursing softly, he reaches for a tissue.

That’s when he sees that the door has been cracked open.

*************************************************************

They go about their usual song and dance. Walk home. Afternoon snack. TV time. Tutoring. Dad comes home. Bedtime.

Papyrus grows to enjoy the tutoring.

“so that’s why humans see the same side of the moon every night,” Sans concludes with a smile.

Papyrus peers over Sans’s shoulder at the doodle he’s drawn of the planet and the moon. “INTERESTING.”

“mmhmm. and we know it isn’t just a phase,” he says with a wink, “because it’s been like this forever.”

He ignores the pun and leans in closer. His skull is next to Sans’s now, only but a few centimeters away. Vaguely he wonders if his brother can hear his soul thrumming in his chest.

“bro?” Sans asks. Is Papyrus imagining it or is his voice softer?

Papyrus grabs Sans’s hand which is still holding the pen aimed at the paper. There’s a definite _squeak_ from Sans and Papyrus isn’t sure he’s heard anything cuter in the world.

“WHAT IF,” Papyrus begins as he drags Sans’s hand across the sketch, “THE MOON WENT THIS WAY INSTEAD?”

He makes the pen draw in the opposite direction, though he focuses on how smooth Sans’s bones are. It’s been some time since he’s last touched him, and he wants to soak it in before he is forced to let go.

Sans takes a while to reply, and when he does, his tone is oddly… neutral. “the direction of the revolution doesn’t matter. it’s just the rate that counts.”

He’s very still, and Papyrus takes it as a hint to stop, though he allows his fingers to brush upwards against Sans’s wrist before withdrawing completely.

“THAT MAKES SENSE! THANK YOU, SANS.” He gives him a bonecrushing hug from behind. “YOU ARE THE BEST BROTHER!”

“heh,” Sans finally utters after a brief, terse silence, “no prob, bro.”

He pats Papyrus’s arms, which are crisscrossed over his sternum.

“love you to the moon and back.”

*************************************************************

One night, when Dad is late to come home, Papyrus swears he hears Sans calling his name. But when he peeks inside the bedroom, he sees his brother thumbing his thick, stout erection. His eyes are closed, and he’s not making any noise.

Papyrus stays to watch. When Sans finally cums, he makes a point to burst into the room only seconds later. His brother scrambles to cover himself, but not before Papyrus sees the thick string of release down his length.

“papyrus, what’re you –?”

“HAS FATHER CALLED YOU? I NEED HIS SIGNATURE FOR MY PARENT-TEACHER CONFERENCE,” Papyrus lies smoothly.

“i –” Sans shakes his head, his blush encompassing his head. “no, i’ll forge it. bring it here.”

As he leaves to gather the paper, Papyrus wonders if the cum will stain the sheets. He wonders what it smells like.

That sinister, curious feeling crawls into his chest again.

*************************************************************

They’re in his room, Sans standing by the racecar bed and Papyrus by the door. It’s dark; neither of them had turned on the lights, as they had only been there to collect Papyrus’s homework. Outside he can hear birdcall as the afternoon sun begins to set.

Papyrus wonders if his soul might burst from his chest.

“sorry, can you say that again?” Sans asks slowly, disbelief laden in his tone. “you want to what?”

“CAN I KISS YOU?” Papyrus repeats, and this time his voice cracks, all the confidence dried up in an instant.

“why?”

Well that really is the question, now isn’t it? Why does he want to kiss Sans, his brother?

Is it because no one else will look his way?

Maybe it’s because Sans is awfully good-looking, so good-looking that Papyrus has conjured up his image more than once when he’s alone at night?

Or perhaps it’s because Sans is the only monster that truly loves him in the world, more than his father who never comes home, never praises him for his accomplishments, or shares in his jokes, or encourages him to be the best he can be, or –

“BECAUSE I AM CURIOUS.”

“curious?” When Papyrus nods, he presses him, “of what?”

“…OF WHAT IT FEELS LIKE,” he supplies.

It’s a half-truth. He does want to know what it feels like, but it implies that he’d be willing to kiss anyone, not just Sans, and there’s no way he could ever think of doing this with someone else.

“bro,” Sans says, and Papyrus can feel himself hunching his shoulders defensively, preparing for the rejection, “we’re… well, we’re family.”

“I KNOW, BUT…” Papyrus sighs shakily. “BUT I JUST WANT TO FEEL IT. NO ONE ELSE WILL DO IT WITH ME, RIGHT?”

There’s a silence, long and pronounced. Papyrus wants nothing more than to sink into the earth, his chest a squiggly mess of nervousness and guilt. He’s terrible for lying, but even more horrible for putting his brother in this position in the first place. He’s the scum of the Underground, really, but he can’t even sleep at night anymore, not with this question, this _curiosity_ plaguing him, interrupting his life every day.

“okay.”

It takes Papyrus a few seconds for the reply to hit him. He snaps his gaze from the floor to Sans and sees him standing there, hand gripping his other arm fiercely as he stares resolutely to the side. His face is blushing furiously, and although he’s clearly flustered, he’s still got that shadow of a smile.

Papyrus wants to ask him if he’s sure, if he heard right, but he’s so scared Sans will back out. And isn’t that selfish of him? But this is all he’s been dreaming of for weeks. No, months.

So although he’s shaking, and his legs are so wobbly he feels like he might trip, he walks the couple meters between them and gently tugs Sans’s face towards him. His brother’s eyelights meet his, and there’s surprise there, and if Papyrus isn’t imagining it, _desire_.

Invigorated, he bends down and pulls Sans in for a kiss. It’s soft, much softer than he would imagine for two skeletons. And warm. Hot, even. Sans is blushing so much that his cheekbones are like flames, warming Papyrus’s already-heated face to even higher temperatures.

When he backs away, Sans’s eyes are closed, and his face is relaxed. The softness in his kiss is reflected there, even his smile gone. There’s something so beautiful about it that Papyrus can’t help himself from bending down again, this time more enthusiastically. His mouth presses against Sans’s, pushing him against the bedframe as his tongue manifests. Sans makes a small noise, and for a moment, Papyrus pauses before he dives into Sans’s mouth. His tongue is there too, wet and warm. Papyrus isn’t quite sure what to do – he’s only ever seen this on human television shows before – but he presses inside anyway, sloppily twirling and dancing against Sans’s tongue. It feels so good, so _right_ , and he just wants to do this forever and ever and –

Sans makes another noise, and this time Papyrus extracts himself. He only now realizes he’s nudged his brother onto the bed, and he’s depravedly fallen – no, _straddled_ – over him.

“OH. I –” Papyrus climbs off him, stunned. “I DID NOT MEAN – S-SORRY.”

His brother is breathing hard. “okay.”

Papyrus isn’t sure what to do. He’s messed up, for sure. Either by doing this the wrong way or doing it at all. The guilt is coming back now, like bile in his throat.

“SANS, I AM SORRY, I –”

“it’s okay,” Sans says again, this time less breathily. “really.”

Is it though? Papyrus doubts that very much.

“that’s all you wanted, right?”

Papyrus pauses. No, he wants more. So much more. He wants to feel Sans beneath him. Wants to bite into his marrow. Wants to feel his conjured flesh against his.

“YES,” he lies.

“okay.” Sans sits up and wipes his face. “guess that’s it then.”

*************************************************************

It happens again. Papyrus has to beg for it, even pulls out the “has no friends” card, but his brother gives in. This time Sans moves his tongue too, and Papyrus knows that the crush is mutual.

*************************************************************

It becomes a routine.

Papyrus and Sans go to school, walk home and grab a snack, watch an afternoon cartoon, and find themselves making out for a while before they break away to do homework together. At first Papyrus has to guilt his brother into it, but eventually Sans stops arguing and acquiesces. With time, he even volunteers for it.

It’s addicting, especially when Papyrus draws out soft groans from Sans when he touches his scapula just right. Or with how their fingers curl together as Papyrus takes Sans’s hand into his own. Or, most intoxicating, when Sans breaks away from the kiss with a lewd, heaving gasp.

In fact, It’s so addicting that there is more than a handful of times where their father nearly catches them at it, even with as late as he is to return from the lab. It’s only because of Sans’s ever-constant vigilance that they’re able to break away in time. He hears Dad stomp up to the front door, cursing the snow, and he teleports away, disheveled and blushing. Father doesn’t notice, of course. He’s too busy grumbling about work.

As much as they are loathe to get caught, they are becoming increasingly daring. Some days Papyrus follows Sans to the school bathrooms and they kiss by the sinks. Other days they’ll go to the park and have a cinnabun. Papyrus will sneak a hand down Sans’s pants, rubbing the curve of his hipbone gently for a couple minutes. Though his brother gets embarrassed and begs Papyrus to stop, he never does anything to prevent it. Secretly he likes it; Papyrus can tell by the way he shivers every time they do anything publicly.

When Papyrus is feeling bravest and thinks their father is too tired to notice, he will sneak into Sans’s room and spend the night with him. It’s all kisses, cuddles, and, although Sans always objects, groping and rubbing. His brother rarely reciprocates, lazybones that he is, but it feels nice when they grind together. Very nice.

It’s things that brothers should never do, but never anything more than that.

Papyrus _wants_ more than that.

His soul aches for intimacy with Sans. He’s everything to him. The smartest, kindest, nicest monster in the Underground. The only one to whom he can divulge everything. His secret-keeper. His rock. His friend. Perhaps even his boyfriend, if he could permit himself that.

And boyfriends have sex. Everyone knows that.

How many times has he stroked himself thinking of Sans begging for Papyrus to take him? How many times has he fantasized milking Sans of every last drop of release? How many times has he hoped that their boneplay, as wonderful as it is, could turn into something more?

But Sans doesn’t want to.

“bro, i…” He swallows, squirming a bit against Papyrus, the blankets shuffling quietly. They’re whispering; it’s late, and Dad is in the room next door. “i don’t know that it’s a good idea. if you, y’know, and then i… well. and then we had a kid –”

“NO, NO, I DO NOT WANT THAT,” Papyrus protests quietly, though he would be lying if he said he hasn’t daydreamed about starting a family. “YOU NEED TO USE YOUR SOUL FOR THAT.”

“i mean, yeah, but –“

“IT WOULD FEEL GOOD, I THINK,” Papyrus urges.

“the bone rubbing feels good, right?”

“FOR YOU, MAYBE,” Papyrus pouts. “BUT I AM THE ONE USUALLY TOUCHING YOU.”

“y-yeah. i thought you liked that.”

“I DO, BUT I WANT MORE THAN THAT.”

“okay. yeah. that’s fine. i’ll, um, touch you more then.”

“SANS,” Papyrus whines, snuggling his head against Sans’s, “I WANT TO REALLY FEEL YOU.”

“what, my bones aren’t me?”

“I WANT TO FEEL YOUR MAGIC.” Papyrus huffs. “I WILL NEVER FEEL ANYONE ELSE’S, WILL I? NOBODY LIKES ME.”

“no, bro, i like you.”

“NOT ENOUGH.”

“paps, we’re… please, we’re bros.”

“WE ARE MORE THAN BROTHERS.”

Sans’s fingers squeeze at Papyrus’s ribcage reflexively.

“papyrus.”

It comes out as a plea. The same tone as when he’s begging for Papyrus to stop in the park.

Papyrus knows he’s won. He kisses his brother’s neck roughly, eliciting a gasp.

“JUST ONCE. JUST THE ONE TIME. NOBODY WILL EVER KNOW.”

The silence stretches. For a moment, Papyrus wonders if he’s wrong, if Sans really _doesn’t_ want to do this. But then –

“okay. god. okay. we can, but not now, please. please, i don’t want dad to hear.”

“I AM NOT AN IDIOT, SANS. TOMORROW. WHEN WE GET HOME.”

“okay. yeah. tomorrow. okay, okay.”

*************************************************************

The next day, Sans conveniently has a project to do with his friends. Papyrus is okay with that. For his brother, he’ll wait until the end of time.

*************************************************************

Their project is lengthy. Sans spends the next week at his friend’s house. Papyrus makes do with his hand and his fantasies.

*************************************************************

When Sans finally gets done, Papyrus comes down with a cold. He wants their first time to be special, not disgusting, so he doesn’t even mention it to Sans. For the next week, they don’t even kiss. He doesn’t want to get his brother sick.

*************************************************************

Sans has apparently forgotten about his promise by the time Papyrus makes his move. His brother starts violently as Papyrus kisses his shoulder. There’s confusion, and dare Papyrus say it, _fright_ in his eyes as Papyrus begins to move up his neck. The expression quickly relaxes as Papyrus begins to stroke his ribs.

“papyrus…”

There’s that begging in his voice again. He wants it so bad, and Papyrus can’t let him down. Not his brother. Not his boyfriend.

Papyrus pushes him onto the bed, but otherwise starts off slow. It’s all bone on bone. Fingers grazing, tongues meeting, gentle nips at sensitive areas.

But he is impatient, and after a few minutes, he quickly picks up speed. His fingers find purchase on Sans’s pubis. His brother is so ready, so aching for his touch, that it only takes a few rough strokes to get him to summon his flesh. It’s already dripping. The wetness is slipping down the curves of his magic, certainly staining the sheets. Sans looks at it, dazed, and Papyrus wants to eat that expression up.

He opts for his cunt instead.

It tastes strange. Sharp, acidic, but with a hint of saltiness. And it’s warm, ever so warm. The thing he relishes the most is how Sans is clawing at his head, begging him to stop. It’s cute how much he’s enjoying it.

When Sans reaches his first orgasm, Papyrus delights in the way he goes limp beneath him. It makes it easier to move him as he positions himself at his entrance.

“papyrus, please,” Sans begs. There are tears at his sockets.

“I WILL BE GENTLE,” Papyrus vows as he pats softly at Sans’s conjured flesh.

As his erection presses into the flesh, stretching it beyond what he thinks possible, he knows that he’ll have to break his promise. It’s so hot, so tight, and he wants nothing more than to thrust into Sans over and over, as hard as he can.

So he does.

“papyrus! no! please, slow down!”

Papyrus ignores him, pistoning inside Sans at a breakneck speed. He can’t quite get all the way inside, so he grapples at his brother’s legs and leans over him. The new angle allows him to bury himself to the hilt, and their ribcages are fleshed together, like they are truly one. Sans is still crying, though now much closer. Papyrus pushes against Sans’s mouth, swallowing all his pleas as he pierces his flesh below.

As expected, Sans reaches orgasm first. His flesh tightens and ripples around Papyrus, and it’s enough to send him over the edge only seconds later. He makes sure he’s submerged as deep as he can. To his disappointment, when he pulls out, the sticky release quickly follows, leaking from the abused opening and onto the bed. More stains to join the rest.

“MESSY, MESSY, MESSY,” Papyrus says with a frown.

Sans makes a small noise. It sounds like a sob, though Papyrus can’t quite tell with the way his brother is covering his face. Curses, he shouldn’t have done that. Swallowing, he lies down next to Sans and rubs his hunched shoulders.

“papyrus,” Sans pleas, his voice muffled in his arms.

“I AM SORRY,” Papyrus offers, genuinely upset by his rashness. “SO SORRY.”

Sans turns over and hugs Papyrus, burying his wet face against his chest. His body is trembling so hard, and Papyrus wishes he could go back in time and do things different. Sans hadn’t deserved this treatment, and Papyrus is supposed to be _better_. He coos his brother, rubbing his scapulae soothingly, praising him for doing so well.

Eventually the sobs even out and Sans stills. When Papyrus is sure he is calmer, he reaches beneath his chin and pulls him away from his hideaway. His eyelights are fuzzy, and his sockets are limned with soggy exhaustion.

“I AM SORRY. IT WILL NOT HAPPEN AGAIN.”

Sans nods vacantly. Encouraged, Papyrus kisses the top of his skull.

“NEXT TIME, I PROMISE I WILL BE GENTLE.”


End file.
